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Title: Polymer field-effect transistors
Author: Georgakopoulos, Stamatis
ISNI:       0000 0004 2746 9976
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2012
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High Ionisation Potential (IP) amorphous conjugated polymers are very practical semiconductors and promising candidates for printing applications as they exhibit 1) high air-stability due to the high IP, and 2) reproducible electrical performance due to the uniformity of amorphous morphology. However they generally exhibit low mobilities on the order of 10-3 cm2/Vs and below. This work is based mainly on two high-IP amorphous conjugated polymers poly(indenofluorene-triarylamine) (PIFTAA) and poly(indenofluorene-phenanthrene) (PIFPA). The long term ambient stability of PIFTAA and PIFPA with IPs of 5. 45 eV and 5. 79 eV respectively is characterised in Field-Effect Transistors (FETs) over a period of 4 and 2 months respectively. FET parameters such as the turn-on voltage and subthreshold slope are found to be generally stable, and the charge carrier mobility is found to degrade at an approximate rate of 10% per month, which is amongst the lowest reported values for organic semiconductors. PIFTAA and particularly PIFPA exhibit high field-effect saturation mobilities of 0. 03 - 0. 04 cm2/Vs and 0. 2 - 0. 3 cm2/Vs respectively, which are unusually high for amorphous conjugated polymers. The morphologies are examined by atomic force microscopy, grazing incidence wide angle x-ray scattering, and differential scanning calorimetry, and no evidence of crystallinity is detected, suggesting that the conjugated polymers are indeed amorphous. To investigate charge transport in PIFTAA and PIFPA, FETs of multiple channel lengths are fabricated, providing mobility data for multiple electric fields, and measured over a range of temperatures. In addition to PIFTAA and PIFPA, the measurements are performed on typical amorphous conjugated polymers poly(triarylamine) (PTAA) and poly(indenofluorene-triarylamine-triarylamine) (PIFTAATAA), with mobilities of 0. 003 cm2/Vs and 0. 004 cm2/Vs respectively. The gate voltage dependence of the mobility extracted from FET measurements, as well as the 1/T2 fit of the mobility with temperature is consistent with a Gaussian Density of States. The indenofluorene copolymers PIFTAA, PIFTAATAA, and PIFPA exhibit clear negative electric field dependence of the mobility, signature of high spatial disorder in the polymer films. The temperature dependence of the mobility is fed into the Gaussian Disorder Model, which indicates that the source of the high mobility for PIFPA is mainly strong intermolecular coupling indicated by the high pre-factor mobility as well as low energetic disorder along the path of charge flow. These results challenge the widely accepted concept that high crystallinity is a requirement for mobility exceeding 0. 1 cm2/Vs in organic semiconductors. Finally, a new type of contact-limited transistor is demonstrated with conjugated polymers. Source-Gated Transistors (SGTs) have a similar structure to FETs, and the main difference is the Schottky source/drain-semiconductor contact, which results in depletion of the near-source region of the semiconductor. Consequently, the behaviour of the transistor changes significantly as compared to FETs. SGTs are demonstrated with several electrodeconjugated polymer combinations. SGTs saturate at significantly lower voltages than FETs, and saturation is not lost for short channels and thick insulators. Also, evidence of independence of the current from channel length is observed, which is consistent with contact-based modulation as opposed to FET channel-based modulation. These advantages come at a cost of output current of at least one order of magnitude, while the intrinsic voltage gain is mostly maintained.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available